Subsurface porous rocks are saturated with fluids. The fluids may be water (salt water or fresh water) or hydrocarbons (gas or oil). The resistivity of a formation may depend heavily on the fluid that is contained within the formation. For example, formations containing hydrocarbons or fresh water tend to be much more resistive that formations that contain salt water. As a result, the resistivity of subsurface formations (e.g., porous rocks) may be measured to determine if the formation is likely to contain hydrocarbons.
The resistivity of subsurface formations may be measured using various methods including a method commonly referred to as controlled source electromagnetics (CSEM). CSEM typically uses a finite size transmitter in which a known time-varying current is made to flow in the subsurface formations by a suitable generator or power supply. The transmitter is typically coupled to two electrodes. The two electrodes are electrically connected to one another via the salt water which acts as a conductor. Hence, a circuit is formed carrying a time-varying current generated by the transmitter.
The time-varying circuit produces a time-varying electromagnetic field which according to Faraday's Law produces a voltage, which drives currents in the ground. That is, the time-varying electromagnetic field causes currents to flow in the subsurface formations. The currents in the subsurface formations may produce secondary magnetic fields which are measured by receivers placed on the ocean floor. The resistivity of the subsurface formations lying below the receivers may be inferred from the magnitude of these secondary magnetic fields.
Typically, the transmitter generates an output current at various frequencies to detect subsurface formations at different depths and regions of the subsurface. The frequency range for CSEM is typically between 1/32 Hz to 32 Hz. Different frequencies are required to detect subsurface formations at different depths and regions of the subsurface because, in general, lower frequencies are able to penetrate to greater depths and higher frequencies can provide more response at shallower depths.
Different frequencies may penetrate across subsurface formations in a variety of ways. For example, a vessel towing the transmitter by a line may pass the subsurface formations using a single frequency (e.g., ⅓ Hz) and then make a second pass over the subsurface formations using a second frequency (e.g., 1 Hz). However, having to make multiple passes over the subsurface formations is time consuming and uneconomic.
Alternatively, the transmitter may transmit a complex waveform that can be deconvolved into a number of frequencies, which are often harmonics of the waveform. However, the energy generated at any particular frequency is greatly reduced using this method. By generating less power at any particular frequency, the signal-to-noise ratio is lower thereby making it more difficult to accurately measure the resistivity in the subsurface formations.
In another alternative method, the transmitter may transmit a square wave at a fundamental frequency. The transmitted square wave will produce energy at the fundamental frequency and also at each odd harmonic frequency. For example, a fundamental frequency of 1 Hz will also contain energy at 3 Hz, 5 Hz and beyond. However, the energy contained within the harmonic frequencies is much reduced from the power at the fundamental frequency. Hence, this alternative method also has problems with having a low signal-to-noise ratio thereby making it more difficult to accurately measure the resistivity in the subsurface formations.
Therefore, there is a need in the art for transmitting each of the different frequencies used in CSEM at full power thereby more accurately and efficiently measuring resistivity of subsurface formations at different depths and regions.
It is thus a desire of the present invention to provide a system and method for generating an output current at various frequencies, each at full power, within a time window. It is a still further desire to provide a system and method for accurately determining the resistivity of a subsurface formations at different depths and regions without making multiple passes over the subsurface formations.